Superconducting inductors for SMES systems are large coils that are usually hundreds or thousands of feet in diameter. Such inductors are capable of storing large amounts of energy in magnetic fields surrounding the coils. They are very efficient for these purposes because no energy is lost to resistive heating in the superconducting current path. The inductors are operated at cryogenic temperatures by immersing the superconducting coil into a dewar of cryogenic coolant such as liquid helium. The cryogenic coolant also functions as an electrical insulator between the coil and the dewar holding the coolant.
Every superconducting material has a critical temperature above which the material is no longer superconducting. If a region of a superconducting inductor losses its superconducting property, i.e., becomes normal or "quenches", joule heating of the normal region occurs. If sufficient joule heating occurs, the normal zone propagates, or grows larger. This joule heating can lead to catastrophic damage to the coil because, as the temperature rises, the resistivity of the superconducting material increases rapidly, causing large voltage differentials to develop across portions of the coil. If the voltage differentials become large enough, arcing can occur from the coil to the dewar. In the event of a coolant dump, arcing is even more likely to occur since the coolant pool acts as an insulator between the coil and the dewar. One method of avoiding damage to the coil when a quench zone develops is to quickly dissipate the energy stored in the inductor. This is usually done by means of "dump resistors" that shunt the energy away from the quench zone.
The rate at which energy can be removed from the inductor at its terminals is limited by the voltage rating of the SMES system for reasons described below. The voltage rating depends upon, among other things, the electrical insulation provided by the coolant pool in which the coil is immersed. When the voltage on the coil exceeds the voltage rating, there is a possibility of uncontrolled arcing and catastrophic damage to the coil.
The rate at which energy (i.e., power) is drained from the inductor at its terminals is equal to the voltage (V) at the terminals times the current (I) through the inductor, or: EQU power=V*I,
where V is the voltage across the inductor terminals and I is the current through the inductor. As the inductor is discharged, i.e., as the energy is drained, the current decreases. Therefore, to remove energy at a constant rate, voltage must increase. This means that the energy cannot usually be drained from the inductor at its terminals quickly enough to avoid arcing between the inductor and the dewar. Discharging through its terminals usually takes hours because of maximum voltage specifications. But, to avoid damage, the inductor must be discharged in a period of minutes.
For quick discharging at points other than at the terminals, SMES coils are usually designed with some type of internal dump resistors. During a coolant dump, the cryogenic coolant is drained and usually replaced with a warm gas, thus allowing the coil to quench. The coil is designed so that when the superconductor quenches, the current shifts to a relatively massive conducting, but not superconducting, structure which is parallel to and physically supports the superconductor. This massive structure serves as the internal dump resistor. For example, in the case of FIG. 6, 42 represents the superconductors and 44 represents the massive supporting structure which functions as the dump resistor. Structure 44 has enough mass to dissipate all of the electrical energy without an excessive rise in temperature. When the current shifts to this structure, however, the system becomes very lossy, with resistance and inductance distributed along the length of the coil. The resistive voltage generated as the inductor discharges to the structure is partially offset by an opposing voltage, or back EMF, generated by the inductor. This cancelling of the resistive and inductive voltages along the length of the inductor allows energy dissipation at a much higher power than would be possible if the dump resistor was external to the inductor. This technique of dissipating the energy into the surrounding structure is referred to as an internal energy dump.
Although the voltages on the coil are orders of magnitude lower during an internal energy dump than when draining energy externally, arcing may result. During normal operation the coil is insulated by the cryogenic cooling fluid, which, as mentioned, may be liquid helium. Although liquid helium is a good electrical insulator, during a coolant dump the liquid helium is usually replaced with warmer gas, for example gaseous helium, which is a much poorer insulator. The voltages on the coil during a coolant dump often exceed the insulating capabilities of gaseous helium. To avoid arcing through the gaseous helium, the voltages on the coil may be decreased by placing "shorting" switches periodically, or at fixed intervals, along the coil. The result is that rather than one large LR circuit there are a number of smaller LR circuits all discharging in parallel. The shorting switches, when closed, serve to provide a controlled current path between the coil and some fixed voltage point, for example a bus bar.
FIG. 3 schematically illustrates the aforementioned shorting switch placement as known in the prior art. The peak voltage in the circuit of FIG. 3 is approximately inversely proportional to the square of the number of shorting switches. These shorting switches are critical to the survival of the inductor. If they do not close prior to the dump, the coil's maximum voltage rating may be exceeded and arcing may occur. Means are therefore usually provided to ensure that arcing occurs at known locations selected to minimize damage, and to ensure that they are extinguished as rapidly as possible.
There must be some means for actuating the shorting switches. The prior art teaches a variety of means for actuating shorting switches that are internal to the dewar. These include: (1) pushrods passing through the dewar, where the motive agent is external to the dewar; (2) an electrically activated solenoid and plunger; (3) hydraulics, with hydraulic pressure supplied by a line passing through the dewar; and, (4) hydraulics, with hydraulic pressure supplied by boiling a cryogenic liquid in the hydraulic system internal to the dewar. The liquid is boiled by resistive heating.
All of the above means require penetration of the dewar so that the energy can be transferred into the dewar to mechanically actuate the shorting switches. Moreover, these require either operator or control system intervention to initiate mechanical motion at the start of a coolant dump.
Other known means for controlling arcing in SMES systems include the utilization of spark gaps, which do not require mechanical motion. However the use of spark gaps creates a number of problems for this application. First, known spark gaps cannot handle the high energy involved in large SMES systems. Second, known spark gaps would necessarily have sacrificial contacts that would vaporize, contaminating the local area of the coil. Third, known spark gaps would become very hot, overheating the local region of the coil.
It is therefore desirable to provide a simple and reliable apparatus and method for automatically closing the shorting switches without operator intervention whenever the cryogenic coolant is dumped by means which do not penetrate the dewar. The present invention achieves this goal.